1. Technical Field
The present invention relates to a purification catalyst for exhaust gas under a high oxygen concentration atmosphere, and especially relates to a purification catalyst for diesel exhaust gas.
2. Related Art
The present applicant formerly proposed a purification catalyst for exhaust gas comprising zeolite carrying Pt and cerium oxide as a purification catalyst for exhaust gas for improving NOx (nitrogen oxides) purification performance (Japanese Patent Unexamined (KOKAI) Publication No.131838/1996). In this case, Pt is a metal for the catalyst, and demonstrates abilities for oxidizing and reducing the exhaust gas. The ability to oxidize in Pt contributes to oxidation reactions of HC (hydrocarbon)+O2xe2x86x92H2O+CO2 and CO+O2xe2x86x92CO2. The ability to reduce in Pt contributes to a reduction reaction of NOxe2x86x92N2 by adsorbing NO in a theoretical air fuel ratio, and contributes to an oxidation reaction of NO+O2xe2x86x92NO2 and a reduction reaction of NO2+HC+O2xe2x86x92N2+CO2+H2O under the high oxygen concentration atmosphere.
Zeolite absorbs unburned HC in the exhaust gas so as to concentrate it, and supplies the unburned HC to Pt as a selective reducing type catalyst. Therefore, it is possible to improve the NOx purification rate under the high oxygen concentration atmosphere. NOx concentration in the vicinity of Pt is increased because the cerium oxide absorbs NOx under the high oxygen concentration atmosphere. Therefore, it is also possible to improve NOx purification performance under the high oxygen concentration atmosphere. The cerium oxide also has an effect of suppressing thermal deterioration of the catalyst.
In the case in which the concentration of the unburned HC as a reducing agent is extremely low at HC/NOxxe2x89xa63 as in diesel engine exhaust, the NOx purification performance of Pt as a catalyst metal is low. Therefore, the reducing ability of Pt must be improved. As a method to improve the reducing ability in Pt, a catalyst in which Pt and Sn are simultaneously carried by a high specific surface carrier (zeolite) is proposed in, for example, Japanese Patent Unexamined (KOKAI) Publication No.71176/1994. In this case, Sn has a spillover effect in which O atoms adsorbed on Pt are transferred to a surface of SnO2 molecules. As a result, if Sn exists in the vicinity of Pt, O atoms which remained in the reduction of NOx are transferred to Sn and are transferred to the zeolite as a HC adsorbent.
However, the transferred O atoms react with a small amount of HC adsorbed on the zeolite, so that the reaction, HC (hydrocarbon)+O2xe2x86x92H2O+CO2, occurs, and the amount of the reducing agent necessary for reducing NOx is decreased.
Therefore, an object of the present invention is to provide a catalyst having high purification performance for the exhaust gas even under a high oxygen concentration atmosphere by suppressing the decrease in the unburned HC in zeolite, to solve the above-mentioned problem.
The present invention provides a purification catalyst for exhaust gas which is used under the high oxygen concentration atmosphere, and in which cerium oxide, zeolite carrying Pt, and tin or tin oxide are mixed so as to exist in the catalyst components.
According to the purification catalyst for exhaust gas of the present invention, even in an environment with a small amount of unburned HC necessary for purifying NOx, like in diesel exhaust gas, it is possible to improve the purification performance for the exhaust gas under a high oxygen concentration atmosphere by retaining the unburned HC in the zeolite at high concentration.
Although the purification catalyst for exhaust gas under a high oxygen concentration atmosphere requires the unburned HC to purify NOx, the unburned HC contained in particularly low amounts diesel exhaust gas. Therefore, zeolite as an HC concentration supply compound, cerium oxide as an NOx concentration supply compound, and Pt as an active species to make the components react, are required in order to improve reactivity of the catalyst.
However, O atoms remain on Pt in the case in which NOx is reduced on Pt by the unburned HC. The remaining O atoms do not substantially move from the surface of Pt, so the adsorption of NOx is rate-determined in the reductive reaction on Pt, and it reduces the reactivity of Pt. Then, it is possible for the O atoms remaining on the Pt to be removed according to a spillover effect by adding tin or tin oxide.
In the addition of tin or tin oxide as a binder, tin or tin oxide is disposed in the interparticle space between cerium oxide, Pt, and zeolite, to provide contacts for tin or tin oxide with cerium oxide, Pt, and zeolite. Therefore, the O atom spilled over from Pt is made to have a composition in which the O atom can move to both cerium oxide and zeolite. The O atoms moved to cerium oxide contribute to an improvement effect in the concentration effect by highly oxidizing NOx, and the O atoms moved to zeolite contributes to an improvement effect in the ability to partially oxidize the unburned HC.
In a technique for adding tin or tin oxide, a technique for carrying by impregnation is also considered. However, according to this technique, when, for example, tin or tin oxide is carried by cerium oxide, contacts of tin or tin oxide with Pt and zeolite are scaled-down, and there is a possibility that tin or tin oxide will be buried in cerium oxide by grain growth in cerium oxide by exposing cerium oxide to high-temperature exhaust gas in the use with this technique. When tin or tin oxide is carried by zeolite, contacts of tin or tin oxide with Pt expand, and the spillover effect appears. However, a distance between tin or tin oxide and cerium oxide as a NOx concentration compound is extended. Therefore, the O atoms can be previously moved to zeolite to be used in the oxidation of HC, so HC as a reducing agent is not supplied, and therefore the improvement effect for NOx purification performance is not observed.
Therefore, it is preferable that tin or tin oxide be added as a binder in the purification catalyst for exhaust gas of the present invention. For the binder of the present invention, it is also possible to use a sol in which tin or tin oxide is mixed with water as it is and a silica binder to which tin or tin oxide is added. Furthermore, it is preferable that the purification catalyst for exhaust gas of the present invention be carried by a honeycomb support by forming the slurry into shape that from the viewpoint of increasing the surface area for exhaust gas. In this case, since the binder also has a role in which the catalyst components are retained by the honeycomb support, it is preferable that tin or tin oxide be added at 5 weight % or more in the catalyst in the present invention. The amount used of tin or tin oxide in this case is the binder quantity which is included in the catalyst component after firing. If the amount of the tin or tin oxide is less than 5 weight %, the catalyst component will peel off after the catalyst component is carried on the honeycomb support.